The trajectories of dust particles ejected from a comet are affected by solar radiation pressure as a function of their ratios of radiation pressure cross section to mass .
Therefore , a study on the orbital evolution of the particles caused by the radiation pressure reveals the physical properties of dust on the surface of the comet nucleus .
In the course of NASA ’ s Deep Impact mission , the ejecta plume evolved under the influence of the radiation pressure .
From the evolution and shape of the plume , we have succeeded in obtaining \beta \approx 0.4 , where \beta is the ratio of the radiation pressure to the solar gravity .
Taking into account \beta \approx 0.4 as well as the observational constraints of a high color temperature and a small silicate-feature strength , dust particles ejected from the surface of comet 9P/Tempel 1 are likely compact dust aggregates of sizes \approx 20 \mu m ( mass \sim 10 ^ { -8 } g ) .
This is comparable to the major dust on the surface of comet 1P/Halley ( \sim 10 \mu m ) inferred from in-situ measurements and theoretical considerations .
Since such dust aggregates with \beta \approx 0.4 must have survived on the surface against jets due to ice sublimation , the temperature of ice in the nucleus must be kept below 145 K , which is much lower than equilibrium temperature determined by solar irradiation and thermal emission .
These facts indicate that 9P/Tempel 1 has a dust mantle composed of 20 \mu m-sized dust aggregates with low thermal conductivities \sim 1 { erg cm } ^ { -1 } { K } ^ { -1 } { s } ^ { -1 } .